Revert "Cleanup enableSyncDefaultUpdate flag (#26236)" (#26528)

This reverts commit c6baac3.

Author: kassens

packages/react-art/src/__tests__/ReactART-test.js

@@ -33,12 +33,16 @@ const ReactTestRenderer = require('react-test-renderer');
 
 // Isolate the noop renderer
 jest.resetModules();
+const ReactNoop = require('react-noop-renderer');
+const Scheduler = require('scheduler');
 
 let Group;
 let Shape;
 let Surface;
 let TestComponent;
 
+let waitFor;
+
 const Missing = {};
 
 function testDOMNodeStructure(domNode, expectedStructure) {
@@ -76,6 +80,8 @@ describe('ReactART', () => {
     Shape = ReactART.Shape;
     Surface = ReactART.Surface;
 
+    ({waitFor} = require('internal-test-utils'));
+
     TestComponent = class extends React.Component {
       group = React.createRef();
 
@@ -357,6 +363,58 @@ describe('ReactART', () => {
     doClick(instance);
     expect(onClick2).toBeCalled();
   });
+
+  // @gate !enableSyncDefaultUpdates
+  it('can concurrently render with a "primary" renderer while sharing context', async () => {
+    const CurrentRendererContext = React.createContext(null);
+
+    function Yield(props) {
+      Scheduler.log(props.value);
+      return null;
+    }
+
+    let ops = [];
+    function LogCurrentRenderer() {
+      return (
+        <CurrentRendererContext.Consumer>
+          {currentRenderer => {
+            ops.push(currentRenderer);
+            return null;
+          }}
+        </CurrentRendererContext.Consumer>
+      );
+    }
+
+    // Using test renderer instead of the DOM renderer here because async
+    // testing APIs for the DOM renderer don't exist.
+    ReactNoop.render(
+      <CurrentRendererContext.Provider value="Test">
+        <Yield value="A" />
+        <Yield value="B" />
+        <LogCurrentRenderer />
+        <Yield value="C" />
+      </CurrentRendererContext.Provider>,
+    );
+
+    await waitFor(['A']);
+
+    ReactDOM.render(
+      <Surface>
+        <LogCurrentRenderer />
+        <CurrentRendererContext.Provider value="ART">
+          <LogCurrentRenderer />
+        </CurrentRendererContext.Provider>
+      </Surface>,
+      container,
+    );
+
+    expect(ops).toEqual([null, 'ART']);
+
+    ops = [];
+    await waitFor(['B', 'C']);
+
+    expect(ops).toEqual(['Test']);
+  });
 });
 
 describe('ReactARTComponents', () => {

packages/react-dom/src/__tests__/ReactDOMNativeEventHeuristic-test.js

@@ -312,7 +312,11 @@ describe('ReactDOMNativeEventHeuristic-test', () => {
       expect(container.textContent).toEqual('not hovered');
 
       await waitFor(['hovered']);
-      expect(container.textContent).toEqual('hovered');
+      if (gate(flags => flags.enableSyncDefaultUpdates)) {
+        expect(container.textContent).toEqual('hovered');
+      } else {
+        expect(container.textContent).toEqual('not hovered');
+      }
     });
     expect(container.textContent).toEqual('hovered');
   });

packages/react-dom/src/__tests__/ReactDOMServerPartialHydration-test.internal.js

@@ -2036,7 +2036,14 @@ describe('ReactDOMServerPartialHydration', () => {
     suspend = true;
 
     await act(async () => {
-      await waitFor(['Before', 'After']);
+      if (gate(flags => flags.enableSyncDefaultUpdates)) {
+        await waitFor(['Before', 'After']);
+      } else {
+        await waitFor(['Before']);
+        // This took a long time to render.
+        Scheduler.unstable_advanceTime(1000);
+        await waitFor(['After']);
+      }
 
       // This will cause us to skip the second row completely.
     });

packages/react-dom/src/events/__tests__/DOMPluginEventSystem-test.internal.js

@@ -1984,9 +1984,13 @@ describe('DOMPluginEventSystem', () => {
             log.length = 0;
 
             // Increase counter
-            React.startTransition(() => {
+            if (gate(flags => flags.enableSyncDefaultUpdates)) {
+              React.startTransition(() => {
+                root.render(<Test counter={1} />);
+              });
+            } else {
               root.render(<Test counter={1} />);
-            });
+            }
             // Yield before committing
             await waitFor(['Test']);
 

packages/react-reconciler/src/ReactFiber.js

@@ -33,6 +33,7 @@ import {
   enableProfilerTimer,
   enableScopeAPI,
   enableLegacyHidden,
+  enableSyncDefaultUpdates,
   allowConcurrentByDefault,
   enableTransitionTracing,
   enableDebugTracing,
@@ -458,9 +459,11 @@ export function createHostRootFiber(
       mode |= StrictLegacyMode | StrictEffectsMode;
     }
     if (
+      // We only use this flag for our repo tests to check both behaviors.
+      // TODO: Flip this flag and rename it something like "forceConcurrentByDefaultForTesting"
+      !enableSyncDefaultUpdates ||
       // Only for internal experiments.
-      allowConcurrentByDefault &&
-      concurrentUpdatesByDefaultOverride
+      (allowConcurrentByDefault && concurrentUpdatesByDefaultOverride)
     ) {
       mode |= ConcurrentUpdatesByDefaultMode;
     }